40-20 Scratch / Dig Neutral Density Filters
Last Updated: April 1, 2025
Description
40-20 Scratch / Dig Neutral Density Filters are optical components designed to reduce the intensity of light without altering its color. These filters are characterized by their surface quality, where the "40-20" specification indicates the allowable size and number of surface defects, such as scratches and digs. Specifically, a 40-20 rating means the average diameter of digs is 0.20 mm and the average length of scratches is 0.40 mm. This specification ensures a certain level of optical clarity and performance, making them suitable for high-precision applications.
Working Principle
Neutral density filters work by either absorbing or reflecting a portion of the incoming light, thereby reducing its intensity. Absorptive neutral density filters achieve this by incorporating certain chemicals into the optical glass, which absorbs specific wavelengths. Reflective neutral density filters, on the other hand, use thin-film interference coatings to reflect part of the light while allowing the rest to pass through. This reduction in light intensity is useful for controlling exposure in various optical systems, allowing for greater flexibility in managing light conditions without affecting the spectral characteristics of the light.
Applications
40-20 Scratch / Dig Neutral Density Filters are particularly useful in applications requiring precise control over light intensity. They are commonly used in laser systems, where maintaining a specific light intensity is crucial for accurate measurements and experiments. These filters are also employed in high-speed photography and long-exposure imaging, where controlling the amount of light entering the camera is essential to prevent overexposure and achieve the desired depth of field or motion effects.
Advantages over other Neutral Density Filters
The 40-20 scratch/dig specification provides a higher level of surface quality compared to lower-rated filters, such as 60-40 or 80-50. This makes them more suitable for high-performance applications where optical clarity and minimal surface defects are critical. The reduced number and size of scratches and digs help maintain the integrity of the optical path, ensuring more accurate and reliable results in sensitive optical systems.
Limitations
One limitation of 40-20 Scratch / Dig Neutral Density Filters is their sensitivity to surface defects, which can still occur despite the high specification. These defects can affect the performance of the filter in extremely precise applications. Additionally, absorptive neutral density filters may have a lower damage threshold compared to reflective types, potentially leading to heating issues during prolonged use.
Considerations
When selecting 40-20 Scratch / Dig Neutral Density Filters, it is important to consider factors such as initial cost, which may be higher due to the stringent manufacturing standards required to achieve the 40-20 specification. Operating expenses may also be influenced by the need for careful handling and maintenance to preserve the filter's surface quality. Durability is generally high, but the filters must be stored and used under appropriate conditions to prevent damage. Replacement and maintenance costs should be factored in, especially in environments where the filters are exposed to harsh conditions or frequent use.
from Gurley Precision Instruments
Wide selection of neutral density filters [See More]
- Surface Quality: 40-20 Scratch / Dig
- Filter Shape: Circular
- Neutral Density Filter Type: Reflective
- Filter Diameter, Side Length, or Rectangular Length: 25.4
from Newport MKS
The 5230 Neutral Density Filter is made with an Inconel ® metallic coating on a glass substrates. This 1.0 inch (25.4 mm) diameter ND filter provides 0.04 OD spectrally uniform attenuation over the wavelength range from 400 to 2000 nm. The reflective nature of the coating helps reduce thermal... [See More]
- Surface Quality: 40-20 Scratch / Dig; 40-20 scratch-dig
- Thickness: 1.1
- Filter Diameter, Side Length, or Rectangular Length: 21.5
- Optical Density: 0.0400
from Newport MKS
The 5231 Neutral Density Filter is made with an Inconel ® metallic coating on a glass substrates. This 1.0 inch (25.4 mm) diameter ND filter provides 0.1 OD spectrally uniform attenuation over the wavelength range from 400 to 2000 nm. The reflective nature of the coating helps reduce thermal... [See More]
- Surface Quality: 40-20 Scratch / Dig; 40-20 scratch-dig
- Thickness: 1.1
- Filter Diameter, Side Length, or Rectangular Length: 21.5
- Optical Density: 0.1000
from EKSMA OPTICS
Neutral density (ND) reflective type filters of 1 ’ ’ (25.4 mm) size with optical density that varies from 0.1 to 2.5 are available. Neutral density filters of Corning 7059 glass provide spectrally uniform attenuation from 400 nm to 2000 nm. The reflective coatings enable to reduce... [See More]
- Surface Quality: 40-20 Scratch / Dig
- Filter Shape: Circular
- Neutral Density Filter Type: Reflective
- Filter Diameter, Side Length, or Rectangular Length: 25.4
from Rapid Spectral Solutions
Neutral density filters available in a wide range of optical densities and sizes. These filters are suitable for all general use. [See More]
- Surface Quality: 10-5 Scratch / Dig; 20-10 Scratch / Dig; 40-20 Scratch / Dig; 60-40 Scratch / Dig; 80-50 Scratch / Dig
- Filter Shape: Circular; Square; Rectangular
- Neutral Density Filter Type: Reflective; Absorptive
- Filter Diameter, Side Length, or Rectangular Length: 6.25 to 100
from HG Optronics, Inc.
A Neutral Density filter allows a photographer to control the exposure in an image very easily. The filter stops light reaching the camera sensor, therefore allowing us to leave the camera with a higher aperture for a longer amount of time. HG OPTRONICS neutral-density filter is a filter that... [See More]
- Surface Quality: 40-20 Scratch / Dig; 60-40 Scratch / Dig
- Filter Shape: Circular
from Thorlabs, Inc.
At Thorlabs, we constantly reinforce our desire to serve at the pleasure of our customers. From this comes our desire to be connected to our industry in a way that far exceeds what would typically be expected of a vendor. Thorlabs lives this core principle by seeking to enhance, at every turn, the... [See More]
- Surface Quality: 40-20 Scratch / Dig; 60-40 Scratch / Dig
- Filter Shape: Circular; Rectangular
- Neutral Density Filter Type: Reflective; Absorptive; Variable
- Filter Diameter, Side Length, or Rectangular Length: 12.7 to 100